<!doctype html>
<html>
<head>
<meta charset="utf-8">
<title>旋转的星空</title>
<style type="text/css">
body{background: black;padding: 0;margin: 0; overflow:hidden}
.header{margin: 0 auto;width: 100%;height: 100%;background-color: #000;position: relative;}
</style>
</head>
<body>
<div class="header"><canvas id="canvas"></canvas></div>
<script>
	var canvas = document.getElementById('canvas'),//获取标签
	ctx = canvas.getContext('2d'), 					// 返回幕布
	w = canvas.width = window.innerWidth,			//获取c长度
	h = canvas.height = window.innerHeight,			// 获取高度
	hue = 217,  // 星星的颜色
	hue = parseInt(1000*Math.random()),
	stars = [],
	count = 0,
	maxStars = 1000;//星星数量
	maxStars = parseInt(5000*Math.random());//星星数量
	var canvas2 = document.createElement('canvas'),
	ctx2 = canvas2.getContext('2d');
	canvas2.width = 50;
	canvas2.height = 50;
	var half = canvas2.width / 2,
	// gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half);
	gradient2 = ctx2.createRadialGradient(half, half, 0, half, half, half); // 画个圆
	gradient2.addColorStop(0.025, '#ccc');
	gradient2.addColorStop(0.1, 'hsl(' + hue + ', 61%, 33%)');
	gradient2.addColorStop(0.25, 'hsl(' + hue + ', 64%, 6%)');
	gradient2.addColorStop(1, 'transparent');
	//星星颜色的渐变
	ctx2.fillStyle = gradient2;
	ctx2.beginPath();
	ctx2.arc(half, half, half, 0, Math.PI * 2);
	// 创建一个圆
	ctx2.fill();
	// End cache
	function random(min, max) {
	 	if (arguments.length < 2) {
			max = min;
			min = 0;
	 	}
		if (min > max) {
		 	var hold = max;
			max = min;
			min = hold;
		}
	 	return Math.floor(Math.random() * (max - min + 1)) + min;
	}
	// console.log(Math.max(w, h));
	// var max = Math.max(w, h);
	// console.log(Math.sqrt(max * max + max * max));
	function maxOrbit(x, y) {
	 	var max = Math.max(x, y),
		diameter = Math.round(Math.sqrt(max * max + max * max));
		return diameter / 2;
	 //星星移动范围，值越大范围越小，
	}
	var Star = function() {
		this.orbitRadius = random(maxOrbit(w, h));
		console.log();
		this.radius = random(60, this.orbitRadius) / 8;
		//星星大小
		this.orbitX = w / 2;
		this.orbitY = h / 2;
		this.timePassed = random(0, maxStars);
		this.speed = random(this.orbitRadius) / 50000;
		//星星移动速度
		this.alpha = random(2, 10) / 10;
		count++;
		stars[count] = this;
	}
	Star.prototype.draw = function() {
	 	var x = Math.sin(this.timePassed) * this.orbitRadius + this.orbitX,
		y = Math.cos(this.timePassed) * this.orbitRadius + this.orbitY,
		twinkle = random(10);
	 	if (twinkle === 1 && this.alpha > 0) {
	 		this.alpha -= 0.05;
	 	} else if (twinkle === 2 && this.alpha < 1) {
	 		this.alpha += 0.05;
	 	}
		ctx.globalAlpha = this.alpha;
		ctx.drawImage(canvas2, x - this.radius / 2, y - this.radius / 2, this.radius, this.radius);
		this.timePassed += this.speed;
	}
	for (var i = 0; i < maxStars; i++) {
	 	new Star();
	}
	function animation() {
		ctx.globalCompositeOperation = 'source-over';
		ctx.globalAlpha = 0.5; //尾巴
		ctx.fillStyle = 'hsla(' + hue + ', 64%, 6%, 2)';
		ctx.fillRect(0, 0, w, h)
		ctx.globalCompositeOperation = 'lighter';
	 	for (var i = 1, l = stars.length; i < l; i++) {
			stars[i].draw();
	 	};
		window.requestAnimationFrame(animation);
	}
	animation();
</script>
</body>
</html>